In general, a gel, in a broad sense of the word, is a solidified substance in which colloid particles or polymer solutes lose an independent molecular mobility due to their interactions to form molecular aggregates. When such a substance contains a dispersion medium, the medium serves to inhibit the colloid particles or polymer substances from separating as agglomerated masses so as to maintain the system in a non-fluid semisolid state. Such a system in which a gel-forming substance (dispersed substance) includes a dispersion medium is called lyogel, that is a "gel" in its narrow sense.
In other words, the terminology "gel" in its narrow sense means a two-component disperse system composed of a solid dispersed substance and a liquid dispersing medium, the system as a whole being a non-fluid semisolid and semiliquid substance. Systems containing water as a dispersing medium are called hydrogels, and systems containing organic solvents are called organogels.
The dispersed substance, i.e., a solid supporting the fundamental structure of a gel, is in many cases an aggregate of a polymer having a crosslinked structure on a molecular level or a fine particle level. It is a well known fact that forces of not only first-order bonding (e.g., covalent bonding and inoic bonding) but second-order bonding (e.g., hydrogen bonding and dipole interaction) take part in supporting the three-dimensional gel structure. Base on this fact, an extrmely large number of examples are implicit in the gel of crosslinked polymers containing a dispersion medium. Among the gels of this type, many kinds of hydrogels containing water as a dispersion medium are known, and extensive studies on hydrogels of synthetic polymers have recently been conducted pursuing the possibility of application to medical, sanitary and agricultural fields.
Polymer compounds which can form hydrogel include, for example, natural polymers, e.g., starch, gum arabic, karaya gum, tragacanth gum, pectin, pullulan, arum root, dextran, sodium alginate, amylose, carrageenan, chitin, gelatin, and casein; semisynthetic polymers, e.g., methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxy ethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, propylene glycol alginate; and synthetic polymers, e.g, polyvinyl alcohol and its modification products, polyvinyl pyrrolidone, (sodium) polyacrylate, (sodium) polymethacrylate, polyacrylamide, poly-2-hydroxyethyl methacrylate, poly-N-dimethylaminoethyl methacrylate, polyglutamic acid, polyaminostyrene, polyethylene oxide-polypropylene oxide copolymers, styrene-maleic anhydride (or its Na or NH.sub.4 salt) copolymers, vinyl acetate-crotonic acid copolymers, vinyl acetate-maleic anhydride copolymers, isobutenemaleic anhydride copolymers, polymethacrylic acidpolyvinyl chloride copolymers, hydrolyzates of polyacrylonitrile, high polyelectrolytes, such as polyvinylbenzyltrimethylammonium and polystyrene sulfonic acid (or a Na salt), and their complexes, and hydrophilic polyurethane.
While polymers which can form an organogel have not received as extensive an investigation as the hydrogels, organogels comprising the above-recited polymers capable of forming a hydrogel and a solvent, e.g., alcohols, acetone, and an alcohol-water mixed solvent, have been studied for their various physical properties. Physical properties of a system comprising a vulcanized synthetic rubber and an oil have hitherto been studied as a representative example of organogels. For example, polypropylene fibers as oil absorbents belong to this category. In recent years, from the viewpoint of interpenetrate polymer network (IPN), polymers having a three-dimensional network structure have been a subject of studies as systems of swelling with various organic dispersion media.
Any of these known gel systems consist of two components of a solid dispersed substance and a liquid dispersion medium, the system retention greatly depending on the interaction between these two components. That is, the liquid functions to prevent the polymer network from degradation followed by formation of compact masses, while the polymer network functions to retain the liquid. Therefore, there is no restraint by first-order bonding due to covalent bonding, though these two components are in some secondary interaction with each other.
The two-component system gel is coagulated with the dispersion medium being separated to become a xerogel (coagel), which re-absorbs a solvent and is thus swollen to form a gel. In other words, the dispersion medium of the two-component system gel can make its entrance in and exit from the system. For example, a hydrogel, which contains a large quantity of water, gradually releases its water content on standing in air and finally becomes a xerogel particularly in exceedingly dry air.
It is however very difficult to absolutely dry a hydrogel, and water more or less remains therein due to moisture absorption. This is attributed to strong bonding between the hydrogel-forming high polymer and water. In this connection, water in which any chemical bonding participates is called bound water, otherwise water being called free water. It is the former that makes it difficult to absolutely dry a hydrogel and causes moisture absorption. The same phenomenon can be seen in organogels containing volatile solvents, e.g., acetone, methyl alcohol, ethyl alcohol, and ethyl acetate. It is therefore difficult to maintain the dispersed substance and the dispersion medium of this type of gel in a constant state for a long period of time in an open system.
In the case of oil gels comprising a vulcanized rubber and a large quantity of an oil, evaporation of the dispersion medium does not occur as long as the oil has a high boiling point or a low vapor pressure. Nevertheless, since the bonding between rubber and oil is essentially weak and a large proportion of the oil does not take part in this bonding , the oil easily bleeds out of the system. Therefore, it is difficult to handle these types of gels containing a large quantity of a solvent while maintaining a constant ratio of the dispersed substance and the solvent. In addition, because the solvent sticks to hands on handling, these gels are not suitable as medical or sanitary materials that may be brought into contact or attachment with the human body even for a short time from the standpoint of preservation stability and hygiene.
Quite recent years have seen studies on use of a gel as a contact medium (coupler) for a probe for ultrasonic diagnosis. As is well known, ultrasonic diagnosis is widespread because the apparatus therefor is cheaper than those for any other diagnostic methods and it can be carried out simply without imposing a burden on a patient. The ultrasonic diagnostic apparatuses are classified by scanning mode as a linear type, a convex type, a sector type, and a trapezoid type, which are selected according to the site and purpose of diagnosis. A probe is chosen in agreement with the scanning mode of the apparatus. Of these scanning modes, a mechanical sector scanning system in which ultrasonic waves from a large aperture (a site for sending and receiving ultrasonic waves) are sharply focused on the part to be inspected has made it possible to detect delicate changes of the tissue. While the part which can be inspected by this system varies depending on the frequency employed, the system is greatly effective in making diagnosis particularly of the surficial tissues, such as the mammary gland, the thyroid gland, and the carotid artery.
The probe to be used in the mechanical sector scanning system has a cylindrical form, and the part to be inspected has conventionally been scanned with the probe having fitted at the end thereof a container made of a synthetic resin in which degassed water is sealed so as to follow the shape of the skin (a so-called water bag). However, preparation of degassed water and sealing of the degassed water into the container are very complicated. Further, some air which unavoidably enters into the interface between the tip of the probe and degassed water frequently causes noise or artifacts, resulting in failure to obtain a clear image. Further, it is necessary to apply jelly to the skin in order to prevent formation of an interfacial air layer between the skin and the container.
Under the situation stated above, studies have been made to use a gel as a contact medium which adds a function of acoustic adjustment for obtaining a clearer diagnositic image. Hydrogels so far proposed for this particular use include a gel of a glycerin aqueous solution containing a cellulose ether compound as disclosed in JP-A-61-146234 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); a gel of a high molecular weight polymer, e.g., polyacrylamide, polyvinyl alcohol, and sodium polyacrylate, which exhibits sufficient strength and water retention and has a water content of 70% or more as disclosed in JP-A-59-82838, and a gel comprising polypropylene glycol and polyvinyl pyrrolidone as disclosed in JP-A-59-49750. Recent studies have been directed to a hydrogel comprising a polyvinyl alcohol/polyvinyl pyrrolidone/water system to which a small amount of sulfuric acid is added as reported in Polymer Preprints Japan, Vol. 36, No.3 (1987).
Organogels comprising a vulcanized styrenebutadiene copolymer or a silicone resin to which liquid paraffin is added have partly been put into practical use.
However, use of these gels as a contact medium for ultrasonic diagnostic probes gives rise to various problems. The hydrogels have poor preservation stability, failing to maintain a constant composition because free water, a dispersion medium, is evaporated in air. Besides, hydrogels generally lack extensibility, are liable to suffer damages due to stretching, scratching, and abrasion, and are brittle and easy to break and therefore do not withstand repeated use. When a jelly having a high water content is applied to the skin for the purpose of preventing entrance of air between the gel and the skin to ensure intimate contact, and the gel is used thereon, water resistance of the gel is unreliable. Existence of a large quantity of free water leads to noise generation and frequency dependence. Further, contact media comprising a hydrogel are prepared by swelling a powderous polymer substance with water to form granular masses and joining the interfaces of the polymer masses so as to mold the masses into a larger shape, such as blocks, sheets, profiles, etc. The thus molded articles essentially contain particle-particle boundaries. When light or sound passes through the molded article, such boundaries cause absorption, irregular reflection or multiple reflection of light or sound.
On the other hand, organogels containing an oil are more practical because they have high boiling points and are therefore free from troubles due to evaporation of the dispersion medium. However, since they contain an excess dispersion medium which is weak in bonding to a gel-forming rubber, the excess oil easily and unlimitedly bleeds out. When the organogel is brought into intimate contact with the skin or direct contact with the heart for ultrasonic body section examination during cardiac operations, the oil bled out from the gel remains on the skin or in the body, giving serious problems of safety and an unpleasant feel.
The present invention provides a new type gel which is a one-component system gel. This gel is deviated from a conventional concept in molecular structure (two-component system). Any publication with respect to such a new type gel cannot be found.